Recently, there has been considerable interest within the detergent industry for laundry detergents which are "compact" and therefore, have low dosage volumes. To facilitate production of these so-called low dosage detergents, many attempts have been made to produce high bulk density detergents, for example with a density of 600 g/l or higher. The low dosage detergents are currently in high demand as they conserve resources and can be sold in small packages which are more convenient for consumers.
Generally, there are two primary types of processes by which detergent granules or powders can be prepared. The first type of process involves spray-drying an aqueous detergent slurry in a spray-drying tower to produce highly porous detergent granules. In the second type of process, the various detergent components are dry mixed after which they are agglomerated with a binder such as a nonionic surfactant. In both processes, the most important factors which govern the density of the resulting detergent granules are the density of the various starting materials and their respective chemical composition. These parameters, however, can only be varied within a limited range. Thus, a substantial bulk density increase can only be achieved by additional processing steps which lead to densification of the detergent granules.
There have been many attempts in the art for providing processes which increase the density of detergent granules or powders. Particular attention has been given to densification of spray-dried granules by post tower treatment. For example, Johnson et al, British patent No. 1,517,713 (Unilever) disclose a batch process in which spray-dried or granulated detergent powders containing sodium tripolyphosphate and sodium sulfate are densified and spheronized in a Marumefizer.RTM.. This apparatus comprises a substantially horizontal, roughened, rotatable table positioned within and at the base of a substantially vertical, smooth walled cylinder. This process, however, is essentially a batch process and is therefore less suitable for the large scale production of detergent powders.
More recently, attempts have been made to provide a continuous processes for increasing the density of detergent granules. Typically, such processes require a first apparatus which pulverizes or grinds the granules and a second apparatus which increases the density of the pulverized granules by agglomeration. These processes, although continuous, require two mixer/densifier apparatus to achieve the desired increase in density, thereby rendering the process more expensive and less efficient.
For example, Curtis, European patent application No. 451,894 (Unilever), discloses a process for preparing high density detergent granules by using two mixers in series. In particular, the starting materials are fed into a high speed mixer/densifier after which the materials are fed into a moderate speed mixer/densifier to increase the bulk density further. Thus, Curtis initially requires a high speed mixer/densifier to pulverize the detergent granules and then a second moderate speed mixer/densfier to increase the density to the desired level.
Appel et al, U.S. Pat. No. 5,133,924 (Lever), disclose a similar process for preparing a high bulk density granular detergent. As with Curtis, Appel et al use a first high speed mixer for pulverizing the detergent granules and then a moderate speed mixer to increase the density of the granules by agglomeration to 650 gl. Moreover, the Appel et al process requires that the detergent granules be deformable, a requirement which renders the process less economical. Thus, the processes disclosed by Curtis and Appel et al require at least two densifier apparatus among other parameters to achieve the desired density level of about 650 g/l .
Jacobs et al, U.S. Pat. No. 5,149,455 (Henkel), disclose a process for increasing the bulk density of detergent granules by using a mixer consisting of an elongated mixing drum substantially cylindrical in shape which is equipped with a rotatable shaft carrying impact tools. The Jacobs et al process, however, achieves an increase in density of only from about 50 to 200 g/l by using a single high speed mixer/densifier with a relatively short residence time (30 seconds or less). Moreover, the Jacobs et al process specifically requires the inclusion of nonionic surfactant in the mixer to achieve the desired density levels.
Hollingsworth et al, European Patent 351,937 (Unilever), disclose a process for preparing high bulk density (650 g/l) detergent granules by using batch mixers such as a Lodige FM.TM. mixer. The Hollingsworth et al process is only a batch process which does not facilitate large-scale production as currently required. Moreover, the mixers used in the process are operated at high speeds to achieve relatively short residence times (5 to 8 minutes). While the Hollingsworth et al process provides high density detergent granules by way of a high speed, single-mixer batch process, it would be desirable to have a continuous process in which low or moderate speed mixers can be used to decrease operating costs.
Accordingly, despite the above-described disclosures in the art, it would be desirable to have a process for continuously producing high density detergent agglomerates having a density of at least 650 g/l. It would also be desirable for such a process to be more efficient and economical to facilitate large-scale production of low dosage detergents.